Recently, I've stumbled across an application in the android market, Juice
Devender, that does exactly what it
advertises. The application extends battery life through automated
micromanaging of peripherals and software. This combined with another
application, Juice Plotter,
allows one to actually experiment with different battery preservation
techniques (or different ROMs for rooted phones) and compare the results.

More specifically, the application behaves much like any other power
management software in Linux installations. It controls the frequency scaling
of the processor (if the phone is rooted), it controls networking interfaces
and screens to help conserve power. All of this functionality can easily be
configured and modified for your desired power settings, but the defaults will
provide a significant boost to battery life. This is one more useful utility
in a long list that allows us to get the most out of our phones.